what frequencies of UV break down plastic headlights and can thin coats really

[CopperCone]

Can you really protect a polycarbonate car headlight with a coating? I want to restore some headlights.

All I see on youtube is a big mess arguing about products etc. What mechanisms are at work? I would like to build some kind of tester to evaluate how much different coatings attenuate the supposedly harmful UV. I have a EEPROM eraser UV light I can use.

https://www.amazon.com/UVP-Memorase-Compact-Eraser-Shortwave/dp/B00CYSZMNQ